Add error reporting mechanism for failing Unwind.

Remove the logging of an error if a thread disappears before the unwind
can begin. This can happen, so allow the caller to determine if this
is really a problem worth logging.

Bug: 27449879

(cherry picked from commit 206a3b97)

Change-Id: If9e7cfeb6eb7b122679a734c1a9eacee8354ef18